1. Field of the Invention
The present invention relates to a method for forming contacts in a semiconductor device, and more particularly to a method for forming contacts, capable of minimizing the formation of poor contacts by utilizing the phenomenon of deposition difference of tungsten (W) between P.sup.+ contacts and N.sup.+ contacts.
2. Description of the Prior Art
Generally, the aspect ratio of a contact hole in a semiconductor device is generally increased as the semiconductor device has a higher integration degree. This is because the contact hole has a relatively increased depth due to a decrease in the width thereof. Such an increase in aspect ratio results in a poor contact between conduction wirings formed above and beneath the contact hole.
In order to minimize the generation of poor contacts, a proposal has been made wherein a metal plug is formed in a contact hole prior to the formation of the upper conduction wiring. The formation of the metal plug is typically achieved by filling tungsten in the contact hole using a deposition method. For depositing the tungsten on the inner surface of the contact hole, a chemical reactor containing a reacting gas mixture of WF.sub.6 and H.sub.2 or WF.sub.6 and SiH.sub.4 is used.
The WF.sub.6 component of the reacting gas mixture of WF.sub.6 and H.sub.2 or WF.sub.6 and SiH.sub.4 reacts preferentially with silicon atoms on a silicon substrate surface exposed by the contact hole, as expressed by the following reaction (1), thereby depositing tungsten on the silicon substrate surface consuming the silicon atoms: EQU 2WF.sub.6 +3Si.fwdarw.2W+3SiF.sub.4 ( 1)
Thereafter, the components WF.sub.6 and H.sub.2 or the components WF.sub.6 and SiH.sub.4 of the reacting gas mixture react with each other, as expressed by the following reactions (2) and (3), thereby sufficiently depositing tungsten on the inner surface of the contact hole to form a metal plug: EQU WF.sub.6 +3H.sub.2 .fwdarw.tungsten+6HF (2) EQU 2WF.sub.6 +3SiH.sub.4 .fwdarw.2W+3SiF.sub.4 +6H.sub.2 ( 3)
Where the tungsten is deposited in accordance with the reaction of formula (1), however, the silicon substrate surface may be excessively eroded. There may also be a problem that the tungsten is deposited in the form of a tunnel. If the silicon substrate is excessively eroded during the procedure expressed in formula (1), the electrical characteristic of the silicon substrate is seriously affected resulting in failure of the proper functioning of the semiconductor device.
In particular, where tungsten is deposited at P.sup.+ and N.sup.+ impurity-diffused regions under a high temperature condition such as above 300.degree. C., for example, the eroding phenomenon is more severe at the N.sup.+ impurity-diffused region than at the P.sup.+ impurity-diffused region. On the other hand, where it is attempted to deposit tungsten at P.sup.+ and N.sup.+ impurity-diffused regions under a low temperature condition such as below 300.degree. C., for example, the tungsten plug deposited on the P.sup.+ impurity-diffused region is nonexistent or has a thickness smaller than the tungsten plug deposited on the N.sup.+ impurity-diffused region. As a result, a poor contact may be formed upon depositing aluminum at a subsequent step since the thickness of tungsten deposited at both P.sup.+ and N.sup.+ impurity-diffused regions are different from each other. This is because the tungsten plug formed at the P.sup.+ impurity-diffused region is grown at a lower rate than the tungsten plug formed at the N.sup.+ impurity-diffused region. Although the reason why tungsten is not grown on the P.sup.+ impurity-diffused region at a lower temperature such as below 300.degree. C. is not clear, many investigations show poor or no deposition of tungsten on the P.sup.+ impurity-diffused region at such a lower temperature. It is believed that such difference of the deposition depth of tungsten will be caused by the difference of eroding depth of the silicon substrate between P.sup.+ and N.sup.+ impurity-diffused regions during the course of reaction expressed in formula (1).
It is more desirable to deposit tungsten on the P.sup.+ and N.sup.+ impurity-diffused regions without eroding the silicon substrate in order to avoid any degradation in the electrical property of the silicon substrate. However, there has not been proposed any method to deposit tungsten on the P.sup.+ and N.sup.+ impurity-diffused regions without eroding the silicon substrate. Therefore, it is more desirable to deposit the tungsten on the P.sup.+ and N.sup.+ impurity-diffused regions with minimum eroding of the silicon substrate at the state of the art.
In order to prevent such a problem, the contact hole formation and the tungsten deposition are conventionally implemented for the N.sup.+ and P.sup.+ impurity-diffused regions respectively so as to form uniform metal plugs at the impurity-diffused regions. As a result, the conventional method of forming metal plugs involves the problem that masking and etching steps are repeated twice.